Field of the Invention
The present disclosure relates to a technology for random access control and resources allocation in a wireless communication system.
Related Art
A next generation wireless communication system is expected to enable person-to-thing and thing-to-thing connection over a network through Machine-to-Machine (M2M) communication without human interruption. It is also expected that new services, such as smart grid or Internet of Things (IoT), can be introduced. In a new-generation network, a large number of Machine Type Communication (MTC) devices are expected to connect eNodeB and MTC devices employing energy harvesting would be introduced. Thus, to support communication between devices, there is need of a method for access control and random access resource allocation for a purpose of management of network loads.
The Third Generation Partnership Project (3GPP) which has led standardization of new generation cellular network technologies, has been making efforts to standardize MTC technologies. In order to deal with the surge of network loads occurring in a random access procedure due to access attempts made by numerous MTC devices, the 3GPP has standardized an Enhanced Access Barring (EAB) technology.
FIG. 1 shows a random access procedure in an existing wireless communication system. The random access procedure in the 3GPP standard includes four steps: preamble transmission, Random Access Response (RAR) transmission, scheduled transmission, and contention resolution. In the step for preamble transmission, an MTC device attempts a random access by transmitting a preamble to eNodeB. To transmit a preamble, an MTC device has to normally receive system information broadcasted by eNodeB. The system information broadcasted by eNodeB includes information for generating a preamble to be transmitted by the MTC device (that is, preamble generation information), and Physical Random Access Channel (PRACH) subframe configuration information. A preamble is based on Zadoff-Chu sequence, so root information required for generating Zadoff-Chu sequence is included in information for generating the preamble. The MTC device may generate up to 64 preambles. The PRACH subframe configuration information indicates a location of each subframe including a PRACH within a frame, and the number of such subframes. Up to 10 PRACH subframes may be configured in one frame based on the PRACH subframe configuration information.
The MTC device selects any one of available PRACH subframes based on the received PRACH subframe configuration information, randomly selects one preamble from the selected subframe based on preamble generation information, and transmits the selected preamble. In the step for RAR, the MTC device receives an RAR message from eNodeB. The RAR message includes Cell Radio Network Temporary Identities (C-RNTI) information, preamble identifier (ID), timing advance information, and uplink transmission resource allocation information. Uplink transmission resources are allocated for uplink transmission for a successfully received preamble. In a case where the RAR message includes preamble ID corresponding to the transmitted preamble, the MTC device determines that the transmitted preamble has been successfully detected by eNodeB, and performs the step of scheduled transmission. Alternatively, in a case where the RAR message does not include preamble ID corresponding to the transmitted preamble, the MTC device determines that it has failed in a random access procedure, and waits to re-perform the step of preamble transmission. In the step of scheduled transmission, the MTC device transmits a message through scheduled transmission based on the uplink resource allocation information included in the RAR message. If successfully receiving the message in the uplink resources, eNodeB transmits a contention resolution message. If the MTC device receives the contention resolution message, it determines that the random access procedure has succeeded. In a case where a plurality of MTC devices selects and transmits the same PRACH subframe and preamble, receives a RAR message, and perform scheduled transmission based on the received RAR message, there may be collision between messages transmitted in uplink resources, so that the corresponding MTC devices end up failing in the random access procedure.
The EAB technology is an access control technology for relieving network loads occurring in a random access procedure. That is, when multiple MTC devices perform random access procedure, the EAB technology aims at preventing an increase in a failure probability of a random access procedure in a case where a plurality MTC devices selects the same PRACH subframe and preamble. For access control of an MTC device based on the EAB technology, eNodeB broadcasts system information including an access barring factor which has a value between 0 and 1. When receiving the system information including an access barring factor, the MTC device selects a random value in a range between 0 and 1, and compare the selected value with the received access barring factor. In a case where the selected value is smaller than the access barring factor, the MTC device performs a random access procedure. Alternatively, in a case where the selected value is equal to or greater than the access barring factor, the MTC device waits without performing the random access procedure. By controlling probabilistically an access of an MTC device by means of an access barring factor EAB, eNodeB is capable of controlling the number of MTC devices participating in the random access procedure.
However, the related art does not provide a specific method for determining information of appropriate PRACH subframe constitution, preamble generation information, and an access barring factor. In order to maximize efficiency of random access resources while satisfying a delay requirement in various services, it is required to determine optimal PRACH subframe constitution information, preamble generation information, and an access barring factor.
A result of multiplication of the number of available PRACH subframes by the number of available preambles is a Random Access Opportunity (RAO) within one frame, and the RAO may determine the maximum number of MTC devices capable of succeeding in a random access procedure in one frame. In addition, the access barring factor may be used to appropriately control the number of MTC devices attempting a random access procedure. If the number of MTC devices attempting a random access procedure is not appropriate compared to the number of RAOs allocated by eNodeB, random access efficiency may be degraded, and, in turn, access delay of MTC devices may increase. Further, the degradation of random access efficiency results in an increase of attempts for random access of MTC devices, thereby leading to an increase in energy consumption. As a result, it may interrupt introduction of MTC devices, such as energy harvesting MTC devices, which are sensitive to energy consumption. Therefore, in order to maximize random access efficiency while satisfying a delay requirement, it need to consider a method for access control based on an access barring factor and a method for random access resource allocation together, which provides RAOs.